The present invention relates to a CVD coating process for depositing α-Al2O3 layers at low temperatures as well as to a coated cutting tool for chip forming machining. The coated cutting tool includes at least one Al2O3-layer deposited according to the present process. The coated tool shows improved toughness behavior when used in interrupted cutting operations and improved wear resistance if the Al2O3 layer is deposited onto a PVD-precoated tool.
Cemented carbide cutting tools coated with various types of hard layers like TiC, TiCN, TiN and Al2O3 have been commercially available for years. Such tool coatings are generally built up by several hard layers in a multilayer structure. The sequence and the thickness of the individual layers are carefully chosen to suit different cutting applications and work-piece materials, e.g. cast iron and stainless steel.
Tool coatings are most frequently deposited by Chemical Vapor Deposition (CVD) or Physical Vapor Deposition (PVD) techniques. In some rare cases also Plasma Assisted Chemical Vapor Deposition (PACVD) has been used. The CVD technique employed for coating cemented carbide tools is conducted at a rather high temperature, from about 880 to about 1000° C. Due to this high deposition temperature and to a mismatch in thermal expansion coefficient between the deposited coating materials and the cemented carbide tool, CVD produces coatings with cooling cracks and tensile stresses. The PVD technique runs at a significantly lower temperature from about 450 to about 700° C. and it is performed under ion bombardment leading to high compressive stresses in the coating and no cooling cracks. Because of these process differences, CVD-coated tools are more brittle and thereby possess inferior toughness behavior compared to PVD coated tools.
With the CVD-technique it is possible to deposit many hard and wear resistant coating materials like Al2O3, TiC, Ti(C,N), TiN, TiCxNyOz and ZrO2. The microstructure and thereby the properties of these coatings can be altered quite considerably by varying the deposition conditions. If the standard CVD deposition temperature could be decreased significantly, an increased toughness of the coated tool would be expected.
A noticeable improvement in performance of CVD-coated tools came about when the MTCVD (Moderate Temperature CVD)-technique began to come into the tool industry from about 5 to about 10 years ago. An improvement in the toughness behavior of the tool was obtained. Today the majority of tool producers use this technique. Unfortunately the MTCVD-technique is limited only to fabrication of Ti(C,N)-layers. The deposition process here takes place at temperatures in the range from about 700 to about 900° C. It uses a gas mixture of CH3CN, TiCl4 and H2.
It is generally accepted that modern tool coatings also should include at least one layer of Al2O3 in order to achieve high crater wear resistance. Hence, it would be desirable if also high quality Al2O3 layers could be deposited by a CVD-process at a temperature in the range similar to that of the MTCVD TiCN-process and closer to the PVD-process temperatures if combined PVD-CVD coatings are desired.
It is well known that Al2O3 crystallises in several different phases: α, κ, γ, δ, θ etc. The most common CVD deposition temperature for Al2O3 is in the range from about 980 to about 1050° C. At these temperatures both singlephase κ-Al2O3 and singlephase α-Al2O3 can be produced or mixtures thereof. Occasionally also the θ-phase can be present in smaller amounts.
In U.S. Pat. No. 5,674,564 is disclosed a method of growing a fine-grained κ-alumina layer by employing a low deposition temperature and a high concentration of a sulphur compound.
In U.S. Pat. No. 5,487,625 a method is disclosed for obtaining a fine grained, (012)-textured α-Al2O3 layer consisting of columnar grains with a small cross section (about 1 μm).
In U.S. Pat. No. 5,766,782 a method is disclosed for obtaining a fine-grained (104)-textured α-Al2O3 layer.
Nanocrystalline α-Al2O3 layers can be deposited by PVD- and PACVD technique at low temperatures as disclosed in U.S. Pat. Nos. 5,698,314, 6,139,921 and 5,516,588. However these techniques are much more technically complicated, process sensitive and have less throwing power than the CVD-technique when used for depositing α-Al2O3 
The κ-Al2O3-, γ-Al2O3- and α-Al2O3-layers have slightly different wear properties when cutting different materials. Broadly speaking, the α-phase is preferred when cutting cast iron while the κ-phase is more often used when cutting low carbon steels. It would also be desirable to be able to produce α-Al2O3-layers at temperatures e g <700° C. that e g can be combined with MTCVD Ti(C,N)-layers or even can be deposited onto PVD-coated layers. Low temperature processes for κ-Al2O3 and γ-Al2O3 are disclosed in U.S. Pat. No. 5,674,564 and in EP-A-1122334. Deposition temperatures in the ranges of from about 800 to about 950° C. and from about 700 to about 900° C. are disclosed.
In DE-A-101 15 390 a coating is disclosed consisting of a PVD-coated inner layer with a top layer of Al2O3 deposited by the CVD-technique at a medium temperature. The Al2O3-layer can be essentially any of the modifications: κ, α, δ and amorphous. A temperature range of from about 700 to about 850° C. is claimed for the deposition process. However, no method for depositing the α-Al2O3 phase at temperatures less than 850° C. is disclosed.
Since α-Al2O3 is the high temperature stable aluminium oxide phase, one would not expect it to be formed at temperatures <800° C. EP-A-1122334 and U.S. Pat. No. 5,674,564 point toward the reasonable assumption that only the metastable phases are possible to be obtained at these low temperatures. So far there have not been any reports on a CVD-process capable of depositing well-crystalline α-Al2O3 at temperatures <800° C. that can be used as a tool coating. However, low temperature Al2O3 CVD-processes using Al-metallo-organic compounds have been reported. Such coatings are generally impure and possess no or low crystallinity and hence are not suitable as tool coatings.
The lifetime and the performance of a coated cutting tool are closely related to the method by which the coating is produced. As mentioned above, high temperature deposition processes generally give cutting tools with lower toughness behavior compared to coatings deposited at lower temperatures. This is due to many factors like differences in the number of cooling cracks formed in the coating, differences in the tensile stress state, influence of the process on the cemented carbide tool body e g degree of decarburisation and degree of diffusion of elements from the cemented carbide into the coating.
On the other hand high temperature deposition processes generally give better coating adhesion due to a substantial interdiffusion of materials from the tool body into the growing coating.
However, there are many cutting operations where high toughness of the tool is more important than high coating adhesion. In such cutting operations the tougher PVD coated tools are frequently used.
PVD-coated tools generally lack wear resistance in comparison to CVD-coated tools. If the temperature of the CVD-process could be lowered for all, or at least for the majority of the coating steps then a higher toughness would be expected and such a CVD-coated tool may better complement the pure PVD-tools in operations where both toughness and high wear resistance is required.